I've had a few cars now that have made a slight gurgling sound in the heater core when you first turn the car on. Is that maybe what you're hearing? Usually it was still from a leak, or because I didn't fill the car up with enough coolant when I was bleeding it. First Gear is a pretty good shop from reputation, so I would assume they'd have done everything right.
I'd ask if you're smelling any coolant, but the suggestion will inevitably make you smell coolant. Check the heater hoses, since a 2002 is starting to get up there in years and they're pretty close to the turbo gear. If they're rock hard they need to be replaced.
Not sure how you would check if the heater core itself has a pinhole leak or something. You'd definitely be smelling that, though.